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Hauhs, M; Hornung, U; Lange, H: A Model of Multicomponent Diffusion in a Reactive Acid Soil in Bourgeat,AP; Carasso,C; Luckhaus,S and Mikelic,A (eds.): Proceedings of the Conference "Mathematical Modelling of Flow Through Porous Media" Sant Etienne, World Scientific, Singapore, 231247 (1995)  
Abstract: In structured soils, a considerable amount of soil water can be immobile. As this immobile water is confined to the smallest pore classes, it may be nevertheless in contact with most of the reactive surface areas of the solid phase. A model is proposed for the following general case: i) The mobile soil water is characterized by a relatively low pH and high Al3+ concentrations due to internal or external inputs of acids; ii) the solid phase consists of silicate minerals (e.g. feldspars) that weather irreversibly and relatively independent of pH by releasing base cations, and iii) all soil water is exposed to a high partial pressure of CO2. In the simplest case we model this situation as a onedimensional multicomponent diffusion problem in which the connections to the mobile phase and the solid phase are described by Dirichlet and Neumann boundary conditions, respectively. Precipitation of secondary minerals (Al(OH)3) in the water column is linked to decreased diffusion coefficients of dissolved ions. The resulting set of coupled nonlinear partial differential equations has a nonstandard form, since a reactiondiffusion system is coupled to a condition of electric neutrality everywhere in space. Therefor, special care has to be taken when applying numerical techniques for parabolic systems. Test calculations show that the model exhibits a new mechanism for SO24 accumulation in acid soils that interferes in a nocel way with variations of NO3 in the mobile water. 
